1. Field of the Invention
The present invention relates to ophthalmic lenses and more particularly, to ophthalmic lenses for the compensation of presbyopia.
2. Description of the Prior Art
Presbyopia is a condition characterized by a reduction in a person's ability to focus upon nearby objects, i.e., accommodation. The onset of presbyopia normally occurs at around the age of forty, even in those people having otherwise good health and normal vision. The condition makes near-distance activities, such as reading, typing, and so on more difficult, or even impossible in advanced cases. Close work may be accomplished more comfortably, in most cases, by utilizing simple single-vision positive lenses having a refractive power of one to three diopters. For many years, this refractive power has been “added” to the prescriptions of people having other refractive deficiencies, e.g., myopia, astigmatism, etc., in the form of “bifocals,” or, when accommodation is severely limited, “trifocals”.
An annoying demarcation between the distance and reading portions of these lenses led to development of “blended” lenses. The distance and reading portions of the lenses are artificially obliterated, but nonetheless remain in a form that interferes with a comfortable transfer from distance to reading portions.
A true multifocal lens has a property such that the refractive power varies continuously and monotonically from top to bottom. While a perceived image quality, i.e., acuity, may vary considerably with a direction of view, i.e., horizontal look angle, some useful image quality is available in most areas of the lens. This type of lens has become known as a “progressive addition” lens (PAL), or “progressive multifocal”, “a lens designed to provide correction for more than one viewing distance in which the power changes continuously rather than discretely” (ANSI Z80.1-1999 for Ophthalmics—Prescription Ophthalmic lenses—Recommendations). Many such designs possess a di-polar character, possessing two identifiable areas intended for distance viewing and reading, which are normally connected by a narrow corridor of reasonably good image quality, where power of the lens varies from that required for reading to the distance prescription.
One such design is described by U.S. Pat. No. 5,123,725 to Winthrop, entitled “Progression Addition Spectacle Lens”. A lens design having similar characteristics is documented in U.S. Pat. No. 5,048,945 to Ueno et. al., entitled “Progressive Power Lens”. Although these lenses appear to function in a similar fashion, the derivation of the shape for their active surface is quite different from one another. Likewise for U.S. Pat. No. 4,988,182 to Takahashi.
Multifocal lenses typically achieve their performance by generalizing the well-known bifocal, or trifocal lenses to include a multiplicity of continuous zones of varying refractive power. This is accomplished by making one of the lens surfaces a non-spherical, i.e., aspheric, shape. In most cases, this aspheric surface is mathematically modeled so that its contours may be manipulated and, finally described with great accuracy for manufacturing purposes. Thus, many modern progressive lens designs are based on an application of differential geometry, and some incorporate methods of variational calculus, or a graphical equivalent, to derive progressive surfaces necessary to obtain a desired refractive power distribution that will satisfy functional requirements and boundary conditions. See O. N. Stavroudis, “The Optics of Rays, Wavefronts and Caustics”, Academic Press, 1972.
The differential geometry representation of PAL surfaces provides incomplete information, with the possibility of misleading an analyst about image quality. The surfaces that result from the application of these design techniques may indeed produce power distributions that are sought, albeit in a narrow vertical corridor, but the visual acuity is often compromised by attempts to expand the power range, or to compress it into too short a vertical corridor. The result is that good vision may be obtained only in discrete areas targeted for distance and near viewing, with the remainder of the corridor, and most areas outside the corridor, delivering only marginal image quality.
A mean local curvature of the aspheric surface of the lens may be controlled, within limits imposed by the power distribution requirements and aberration constraints, to achieve a desired variation in diopter power. Aberration content of a progressive addition lens is ordinarily characterized by evaluating a difference in principal curvatures in geodesic orientations at selected points on the lens, and is conventionally expressed solely as astigmatism.
An assumption that the aberration content is pure astigmatism is, of course, a simplistic one. If ray pencils are small, the approximation may be fairly good. If the ray pencils pass into a fully dark-adapted eye, many different aberration components may be present, and it is a summation of these that will determine the ultimate visual acuity when viewing through different sections of the lens.
The characterization of any PAL reduces, naturally, to some physical surface shape. This shape should ideally be continuous, monotonic, and free of severe second and third partial derivatives, otherwise the user will be acutely aware of local variations in both geometric distortion and acuity, and will experience discomfort in extended use. These limitations restrict the power distributions that may be implemented in practice. If the local power of a PAL is required to change rapidly from point to point, severe inflections in the aspheric surface will be present, and the stigmatism of the transmitted ray pencil will be less than ideal in some portions of the lens.
Another aspect of the surface characterization problem is that any such surface must be accurately modeled mathematically in order to be generated and manufactured. Many mathematical representations have been applied to the characterization of PAL surfaces. Most of these have been Cartesian-based, that is, expressed in X-Y coordinates. This is not necessarily bad, but since the eye pupil is a circular aperture, it makes some sense to fit a function to the PAL surface that is based upon polar coordinate geometry. As in many other situations requiring mathematical analysis, matters are made easier by choosing a coordinate system matched to the physical circumstances.
People normally perform reading tasks in a large variety of head and body positions. While certain texts describe “ideal” body geometry for fatigue-free reading, this is rarely realized in practice. Compromises made to achieve a desired power distribution, coupled with a need to accommodate the physiological act of convergence of the two separate visual systems when reading, often results in a need for the user to retrain himself to substitute head movement, when using PALs, for the more natural act of eye movement. Further, variations over a user base of interpupillary distance, center-of-rotation, and other facial characteristics, require that each prescription be custom fitted with great care. These “fitting factors” limit the scope of application and the overall utility of these designs. A design that is not constrained by these fitting factors will find more widespread application, and will be easier to accept in use.